Construction costing

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Throughout the building production process costs will have a major influence when choosing from alternative designs. An excessively high cost may even cause the whole project to be abandoned. In the initial stages, when rough sketches are evaluated, general guideline costs based on building area or volume may be sufficient. In the final design stage, when the farmer has to decide whether or not to proceed with construction, a more detailed cost estimate based on a simplified bill of quantities is usually prepared. A contractor will need the most accurate cost estimate based on a bill of quantities, since his quotation should be low enough to be competitive, but still give him a profit. On large projects, the bill of quantities is also used to determine interim payments for work that has been completed.

Quantity Surveying

The objective of quantity surveying is to provide an accurate bill of quantities, which is a list of the amounts of all materials and labour necessary to complete a construction project. In the simplified version, supplied by the designer with the final design documents, the labour requirement is not detailed. Enough accuracy for the purpose of this bill will be obtained by including labour as a lump sum, number of hours or day's work or as a percentage cost of the building material cost. A bill of quantities for a standard drawing often excludes such operations as site clearance, excavation and fill, and external works, since such quantities may vary greatly from one site to another and therefore be difficult to assess accurately at the time the drawing is completed. Indeed, the bill for a standard drawing may be a mere list of materials, perhaps with a rough estimate of labour added.

To avoid mistakes or omission of any item, sophisticated methods have been developed for quantity surveying of large-scale projects. Farm buildings are normally smaller and far less complex and therefore a simplified procedure will be adequate. Many rules of thumb or conversions have been developed to take into account such factors as cutting waste, difference between nominal and actual size and breakage.

Taking-off

The objective of taking-off is to produce a detailed list of all materials and work. Assessment of the quantities is made from detailed drawings and specifications of the project and done, as far as possible, in the order that the construction of the building will proceed. First items are site clearing, excavation and foundation and final items are finishes and external works. The dimensions of each item are obtained from the drawings and then the quantity is calculated in the units in which it is customarily sold or priced. For example, excavation or fill, concrete, mortar and water would be in cubic metres, aggregates in cubic metres or tonnes, cement and lime in number of bags and many things such as bricks and blocks, windows and doors, building boards and roofing sheets in number of units. Sawn timber is listed in the number of pieces of specific sizes or where that is not necessary, total linear or volume quantities. Round timber will be in number of units of specific cross section and length.

A particular item which occurs in several places in a building, can be noted each time it occurs or the number of units can be totaled at one place. One way of obtaining completeness is to tick off each item on the drawing as it is listed.

Assessment of Labour

Detailed labour requirements to complete the type of construction commonly used in farm buildings may be difficult to find in published sources. This is because the contractors, who have the best knowledge of such data, use them as a means to compete for tenders. Also, most construction companies involved in farm building are too small to employ a quantity surveyor, who could collect the data. Data published by Quantity Surveyors' or Building Contractors' Associations tend to emphasize urban types of construction.

The rough estimates of the labour requirement needed by the designer of farm structures must be obtained through experience and by analyzing a number of projects similar to the one at hand. Where the farmer and the farm labourers construct a building, it is to be expected that the labour requirement will be higher than when skilled construction workers are used. However, the farm labour is available without any extra cash payment and may have few alternative uses during the off season.

Bill of Quantities

The items for a bill of quantities are normally grouped together under headings for either the main operations (excavation, foundation, walling, flooring, roof structure, roofing, finishes, external works) or the trades involved (earth work, masonry, concrete work, carpentry and painting). Work normally carried out by sub-contractors (wiring, plumbing, installation of equipment and furnishing) is listed separately.

The total quantity of each material or volume is transferred from the taking-off sheets to the appropriate heading in the bill of quantities, and while doing so, a percentage allowance for waste and breakage, is normally added. The percentage added will depend on the type of material or volume, but is often taken to be 5 to 15%. To keep a record of the items, they should be ticked off on the takingoff sheets as they are transfered to the bill of quantities.

Labour may be listed under each operation or trade, but in the simplified bill, it is given as a lump sum at the end.

Figure 6.2 Main drawing for poultry house.

Example

Make a bill of quantities for the poultry house illustrated in Figure 6.2. Start with taking-off.

The amount of ingredients can be calculated using the figures in Table 3.14.

The amount of cement and sand for the mortar and plaster can be calculated using the values in Table 3.17.

Roofing

Corrugated steel sheets are laid in 2 rows on each side and the covering width is 533mm per sheet. The length of the roof is 9m.

9000/533=16.9 i.e

17 sheets are required per row or a total of 68.

Roofing Nails

6 nails per m² x 68m² = 408. Since each kg of nails holds about 97, the requirement will be 4.5 kg.

When all requirements are calculated, the amounts are put in the bill of quantities that follows:

Costing

As mentioned in the introduction to this section there is a need to continuously assess the building costs for a proposed structure throughout the planning stages of the building production process. Three levels of accuracy can be distinguished: general guidance cost, specific guidance cost and accurate costing.

In addition, costing is carried out during the construction to ascertain how the project is progressing from a financial point of view and to determine any interim payments to the contractor.

In the post-construction stage the actual cost of the project should be calculated so that a record can be developed which will enable future building work to be accurately costed. Unfortunately this is often neglected by designers and builders of farm structures.

General Guidance Cost

In this case, rough estimates, simply giving the scale of costs, are derived by experience and analysis of a number of other similar projects. For example, if the costs of a number of grain stores are assessed and in each case compared with the capacity of each store in tonnes, then a rough cost for grain stores can be estimated in terms of cost per tonne stored. Hence an estimate can be given for a proposed new grain store if the capacity is known. Similarly, a building for dairy animals can be estimated if an average cost per cow is known from a number of different units.

Further, for particular types of construction, it is possible to get average figures in terms of floor area. This type of estimate is based on a number of projects, some of which may not be directly comparable.

Specific Guidance Costs

By comparing similar projects, it may be possible to get estimates which are reasonably accurate before the time is taken to design the building and work out the bill of quantities. In this case, the costs of other buildings should be assessed in three components:

Established costs, those costs that either have a fixed value or a uniform-unit value regardless of the size of the building. Examples are windows and doors.

Variable costs, those that vary with the size of the building. As the length of a building grows its total cost will grow, but at the same time the unit cost may decrease so that a building that is 50% longer may increase only 40% in cost.

Additional costs, such as fees for consultants, architects, lawyers and accountants. Interest, insurance, fitting costs and losses should also be included.

Therefore, if a number of similar buildings are analyzed, good estimates of each of these types of costs may be obtained and reliable specific guidance costs can be determined.

Table 6.2 Bill of Quantities for Poultry House (see figure 6.2)

Accurate Costing

This is done in conjunction with the bill of quantities. By use of the rate column in the final bill of quantities together with the cost rate for each item, the accurate total cost of a job can be derived. This requires each individual item of material, volume or labour to be costed.

However, many building contractors will, for convenience and to facilitate the calculation of a quotation, derive a cost per quantity of common types of construction. For example, a cost per square metre of concrete block wall will include the cost of the blocks, labour to mix mortar, cost of mortar materials and labour required to lay the blocks. It may even incorporate a factor taking into consideration the average requirement of window and door openings and scaffolding.

To be able to do costing with this degree of detail however, considerable information is needed which is gained from experience and data collected and analyzed over a number of building projects. The unit costs will have to be reviewed continuously or be corrected with an index for building costs.

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